Detecting an analyte using a piezoelectric cantilever sensor

ABSTRACT

A piezoelectric cantilever sensor includes a piezoelectric layer and a non-piezoelectric layer, a portion of which is attached to the piezoelectric layer. In one embodiment, one end of the non-piezoelectric layer extends beyond the end of piezoelectric layer to provide an overhang. The overhang piezoelectric cantilever sensor enables increased sensitivity allowing application of the device in more viscous environments, such as liquid media, as well as application in liquid media at higher flow rates than conventional piezoelectric cantilevers. In another embodiment, the sensor includes first and second bases and at least one of the piezoelectric layer and the non-piezoelectric layer is affixed to each of the first and second bases to form the piezoelectric cantilever sensor. In this embodiment, the sensor is robust and exhibits excellent sensing characteristics in both gaseous and liquid media, even when subjected to relatively high flow rates.

CROSS REFERENCE TO RELATED APPLICATIONS

The instant application is a divisional application of U.S. patentapplication Ser. No. 11/625,919, filed Jan. 23, 2007, currently pending,which is incorporated herein by reference in its entirety. U.S. patentapplication Ser. No. 11/625,919 claims priority to U.S. provisionalpatent application No. 60/761,172, entitled “PIEZOELECTRIC CANTILEVERSENSORS,” filed Jan. 23, 2006, and U.S. provisional patent applicationNo. 60/807,020, entitled “PIEZOELECTRIC CANTILEVER SENSORS,” filed Jul.11, 2006, both of which are hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The technical field generally relates to sensors, and more specificallyrelates to piezoelectric cantilever sensors and to detecting andmeasuring analytes utilizing a piezoelectric cantilever sensor.

BACKGROUND

Cantilever sensors can be broadly divided into two categories, dependingupon dimensions of the sensor: micro-cantilevers and macro-cantilevers.Micro-cantilever sensors can be utilized in both static (bending) modeand dynamic (resonance) mode. In static mode, the deformation of thecantilever arm is measured to determine if an analyte (substance underanalysis) is present. In dynamic mode, a resonance frequency is measuredto determine if an analyte is present. Macro-cantilever sensorstypically are not utilized in the static mode because bending of thecantilever arm is often limited. Macro-cantilever sensors can beutilized under liquid immersion conditions or in a gas or vacuum.Typically, greater sensitivity is achievable when a cantilever sensor isutilized in a gas/vacuum than in a liquid. Liquid dampening tends toadversely affect sensitivity. However, measuring analytes in liquidmedium has many practical applications.

One type of known micro-cantilever sensor is a silicon-basedmicro-cantilever sensor. A typical silicon-based micro-cantilever sensorcomprises a micro-cantilever that acts as a resonator. Themicro-cantilever is driven by an external actuator at the base of themicro-cantilever to generate vibrations in the resonator. Typically, thevibrations are detected by an external optical detector. Onedisadvantage of typical silicon-based micro-cantilevers is the complexexternal optical components required for detection. Further, opticaldetection means disadvantageously limit application of themicro-cantilever sensor to optically clear samples. Another disadvantageis the weight and complexity added to the sensor due to the externalactuator. Yet another disadvantage is that the external actuator can belocated only at the base of the micro-cantilever, which limits itseffectiveness in driving the cantilever's vibration. A furtherdisadvantage of silicon-based micro-cantilever sensors is that they aremechanically fragile. Thus, silicon-based micro-cantilever sensors cannot be used in high liquid flow rate environments. Further, typicalsilicon-based micro-cantilever sensors lose detection sensitivity inliquid media due to viscous damping.

Another type of known cantilever sensor is a quartz-based piezoelectriccantilever sensor. Quartz is a weak piezoelectric, and thus, much likesilicon-based cantilever sensors, quartz-based piezoelectric cantileversensors lose detection sensitivity in liquid media due to viscousdamping. Further, the detection sensitivity of quartz-based sensors islimited by the planar geometry of the sensor.

Conventional piezoelectric cantilevers are known to be fabricated with apiezoelectric layer attached to a non-piezoelectric layer over part orthe entire surface of the piezoelectric layer. In some conventionalpiezoelectric cantilevers, the piezoelectric layer is fixed at one endso that when the piezoelectric material is excited, thenon-piezoelectric layer flexes to accommodate the strain caused in thepiezoelectric material. When the frequency of excitation is the same asthe natural frequency of the underlying mechanical structure, resonanceoccurs. This type of piezoelectric cantilever sensor is known to operateat frequencies lower than about 100 kHz at millimeter size. Currently,higher frequencies are obtainable only by making the cantilever sensorvery short (less than 1.0 mm in length), very narrow (less than 0.1 mmin width), and very thin (less than 100 microns in thickness). However,reducing the dimensions of the cantilever sensor, particularly thewidth, thusly, makes the cantilever sensor less usable in a liquidmedium due to viscous damping. Damping increases inversely with squareof cantilever width.

SUMMARY

A self-exciting and self-sensing piezoelectric cantilever sensingapparatus includes a piezoelectric layer and a non-piezoelectric layerattached to the piezoelectric layer such that a distal end of thenon-piezoelectric layer extends beyond a distal end of the piezoelectriclayer or a distal end of the piezoelectric layer extends beyond a distalend of the non-piezoelectric layer. That is, the piezoelectric layer iscoupled to the non-piezoelectric layer such that the piezoelectric layerand the non-piezoelectric layer are not coextensive In variousconfigurations of the piezoelectric cantilever sensing apparatus, thepiezoelectric layer, the non-piezoelectric layer, or both are anchoredto at least one base. Electrodes are operatively associated with thepiezoelectric layer. The self-exciting, self-sensing piezoelectriccantilever sensor is utilized to sense mass change. To determine themass of an analyte on the sensing apparatus, the resonance frequency ofthe mechanical member of the cantilever sensor is measured. The measuredresonance frequency is compared with a baseline resonance frequency todetermine a difference in frequency. The difference in frequency isindicative of a mass of an analyte on the sensing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, isbetter understood when read in conjunction with the appended drawings.For the purpose of illustrating a self-exciting, self-sensingpiezoelectric cantilever sensor, there is shown in the drawingsexemplary constructions thereof; however, a self-exciting, self-sensingpiezoelectric cantilever sensor is not limited to the specific methodsand instrumentalities disclosed.

FIG. 1 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor.

FIG. 2 is a cross-sectional view of an example self-exciting,self-sensing piezoelectric cantilever sensor depicting electrodeplacement regions for electrodes operationally associated with thepiezoelectric layer.

FIG. 3 is a cross-sectional view of an example self-exciting,self-sensing piezoelectric cantilever sensor showing depicting exampleelectrode placement within a base portion of the self-exciting,self-sensing piezoelectric cantilever sensor.

FIG. 4 is a cross-sectional view of an example self-exciting,self-sensing piezoelectric cantilever sensor showing depicting exampleelectrode placement not within a base portion of the self-exciting,self-sensing piezoelectric cantilever sensor.

FIG. 5 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thedistal end of the piezoelectric layer is flush with the distal end ofthe non-piezoelectric layer.

FIG. 6 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thedistal end of the piezoelectric layer extends beyond the distal end ofthe non-piezoelectric layer and the proximate end of the piezoelectriclayer extends beyond the proximate end of the non-piezoelectric layer.

FIG. 7 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor having twobase portions.

FIG. 8 is an illustration of another example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor, wherein thepiezoelectric layer is not attached to either base portion.

FIG. 9 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor having thepiezoelectric layer anchored at two ends.

FIG. 10 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thepiezoelectric layer comprises two portions, one of which is anchored.

FIG. 11 is another illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thepiezoelectric layer comprises two portions, one of which is anchored.

FIG. 12 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thepiezoelectric layer comprises two portions, neither which is anchored.

FIG. 13 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor having ananchored non-piezoelectric portion and a non-anchored piezoelectricportion.

FIG. 14 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor, wherein thenon-piezoelectric layer is not attached to either base portion.

FIG. 15 is illustration of another example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor wherein thepiezoelectric portion has a different width than the piezoelectricportion.

FIG. 16 is an illustration of an example configuration of aself-exciting, self-sensing piezoelectric cantilever sensor comprising apiezoelectric layer and a non-piezoelectric layer, wherein the width, ofthe piezoelectric layer is less than the width of the non-piezoelectriclayer 16, and the distal end of the piezoelectric layer extends beyondthe distal end of the non-piezoelectric layer and the proximate end ofthe piezoelectric layer extends beyond the proximate end of thenon-piezoelectric layer.

FIG. 17 is a flow diagram of an example process for detecting an analyteutilizing the self-exciting, self-sensing piezoelectric cantileversensor.

FIG. 18 is a plot of an example resonance spectrum of the configurationof the self-exciting, self-sensing piezoelectric cantilever sensordepicted in FIG. 1, operated in air.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A self-exciting, self-sensing piezoelectric cantilever sensor asdescribed herein provides the ability to detect and measure extremelysmall amounts of an analyte. The self-exciting, self-sensingpiezoelectric cantilever sensor can be utilized to detect and measure ananalyte immersed in a liquid and an analyte contained in a gas orvacuum. In various example configurations, the self-exciting,self-sensing piezoelectric cantilever sensor comprises at least onepiezoelectric layer and at least one non-piezoelectric layer, whereinthe piezoelectric layer is coupled to the non-piezoelectric layer suchthat the piezoelectric layer and the non-piezoelectric layer are notcoextensive. The piezoelectric layer, the non-piezoelectric layer, orboth can be coupled to at least one base. The piezoelectric layer andthe non-piezoelectric layer can be of varying widths, lengths, andthicknesses.

The self-exciting, self-sensing piezoelectric cantilever sensor isutilizable to determine the mass of an analyte accumulated thereon. Inan example embodiment, a portion of the self-exciting, self-sensingpiezoelectric cantilever sensor is placed in a medium (e.g., liquid,gas, vacuum). While in the medium, a resonance frequency of theself-exciting, self-sensing piezoelectric cantilever sensor is measuredand compared to a baseline resonance frequency. The difference in themeasured resonance frequency and the baseline resonance frequency isindicative of an amount of mass of analyte accumulated (e.g., bound,adsorbed, absorbed) on the self-exciting, self-sensing piezoelectriccantilever sensor.

Analytes can be directly or indirectly bound to the surface of thenon-piezoelectric portion of the self-exciting, self-sensingpiezoelectric cantilever sensor. Binding of an analyte to thenon-piezoelectric portion of the self-exciting, self-sensingpiezoelectric cantilever sensor results in a change in mass of theself-exciting, self-sensing piezoelectric cantilever sensor, a change instiffness of the self-exciting, self-sensing piezoelectric cantileversensor, or a combination thereof. The changes in mass and/or stiffnessare measurable as changes in resonance frequency, and can be monitoredand measured by an appropriate analysis device, such as an operationalamplifier, an impedance analyzer, a network analyzer, an oscillatorcircuit, or the like, for example. Resonance frequency changes, whereinat least a portion of the self-exciting, self-sensing piezoelectriccantilever sensor is immersed in a liquid, are detectable andmeasurable. Resonance frequency changes, wherein at least a portion ofthe self-exciting, self-sensing piezoelectric cantilever sensor isimmersed in a gas or a vacuum, also are detectable and measurable.

The self-exciting, self-sensing piezoelectric cantilever sensor isoperateable at high frequencies, such as, on the order of 0.1 MHz. to 6MHz, for example. At these high frequencies, a Q factor (the ratio ofthe resonance peak frequency relative to the resonance peak width athalf peak height), on the order of 10 to 100, under liquid immersion isobtainable. The self-exciting, self-sensing piezoelectric cantileversensor is operateable at relative high frequencies in liquid media, gasmedia, and a vacuum. The self-exciting, self-sensing piezoelectriccantilever sensor thus provides extreme sensitivity to mass changes. Theself-exciting, self-sensing piezoelectric cantilever sensor isespecially suitable for analytes that are present at very lowconcentrations in media such as in body fluids, water, and foodmaterials, for example.

The self-exciting, self-sensing piezoelectric cantilever sensordescribed herein provides the ability to detect changes in massaccumulated thereon as small as 100 attogram/Hz (100×10⁻¹⁸ grams/Hertz)or less when immersed in a liquid media. Thus, with respect to detectingchanges in mass, the self-exciting, self-sensing piezoelectriccantilever sensor is approximately 1 million times more sensitive than aquartz crystal micro-cantilever sensor, approximate 100,000 times moresensitive than standard analytical instruments, and about 10,000 timesmore sensitive than conventional, three-layer piezoelectric cantileverdesigns.

The self-exciting, self-sensing piezoelectric cantilever sensor permitsdetection of extremely small concentrations of analyte that bind to thenon-piezoelectric portion thereof. Utilizing the self-exciting,self-sensing piezoelectric cantilever sensor, pathogens and proteins aredetectable at concentrations as low as a few pathogens/mL and, forproteins of average size (60 kilo-Daltons, kDa), at less than 1pathogen/mL. Furthermore, any analyte that binds to an organic orinorganic functional group on the non-piezoelectric portion isdetectable. The self-exciting, self-sensing piezoelectric cantileversensor is operable in media having relatively high flow rates. Thepiezoelectric cantilevers sensors is operable in media having flow ratesof 0.5 to 10.0 mL/minute, which is approximately 1000 times the flowrate used successfully with known bending mode micro-cantilevers.

Various example applications of the piezoelectric cantilever include thedetection of bioterrorism agents, such as Bacillus anthracis, thedetection of food-borne pathogens, such as E. coli, the detection ofpathogens in food and water, the detection of certain cell types in bodyfluids (e.g., circulating tumor cells), the detection of biomarkers inbody fluids (e.g., proteins that mark specificpathophysiology-alpha-fetoprotein, beta-2-microglobulin, bladder tumorantigen, breast cancer marker CA-15-3, and other CAs (cancer antigens),calcitonin, carcinoembryonic antigen, and others), the detection ofmarkers of explosives such as trinitrotoluene, the presence ofdinitrotoluene, and the detection of airborne and waterborne toxins. Theself-exciting, self-sensing piezoelectric cantilever sensor also can beused for the detection of biological entities at picogram levels, andfor the detection of protein-protein interactions, both steady state andkinetic.

Pathogens, such as E-coli for example, are detectable utilizing theself-exciting, self-sensing piezoelectric cantilever sensor. Detectionof a model protein, lipoprotein, DNA, and/or RNA at a concentration 1.0femtogram per mL (10⁻¹⁵ grams) and pathogens at 1 pathogen/mL,respectively is achievable by measuring directly in liquid using theself-exciting, self-sensing piezoelectric cantilever sensor immobilizedwith antibodies specific to the target analyte at a frequency of about 1to 2 MHz. The self-exciting, self-sensing piezoelectric cantileversensor is capable of detecting a target analyte without false positivesor negatives even when contaminating entities are present. Theself-exciting, self-sensing piezoelectric cantilever sensor describedherein is particularly advantageous when utilized with a raw sample, andno preparation, concentrating step, and/or enrichment of any type.Detection of an analyte utilizing the self-exciting, self-sensingpiezoelectric cantilever sensor can be conducted directly in raw samplesunder flow conditions, such as 0.5 to 10.0 mL/minute for example. Ifclean samples are available, such as in a laboratory environment,detection at 1 femtogram/mL is achievable. This sensitivity isapproximately 100 times more sensitive than the sensitivity associatedwith known optical techniques.

As described below, the sensitivity of the self-exciting, self-sensingpiezoelectric cantilever sensor is due in part to the geometric designthereof. The relative lengths and widths of the piezoelectric andnon-piezoelectric layers of the self-exciting, self-sensingpiezoelectric cantilever sensor determine the sensitivity, and also theshape of the peak of the frequency spectrum provided by theself-exciting, self-sensing piezoelectric cantilever sensor. Asdescribed in more detail below, the self-exciting, self-sensingpiezoelectric cantilever sensor comprises a piezoelectric layer and anon-piezoelectric layer coupled together such that a portion of thepiezoelectric layer extends beyond the non-piezoelectric layer, or aportion of the non-piezoelectric layer extends beyond the piezoelectriclayer, or a combination thereof. Thus, the piezoelectric layer and thenon-piezoelectric layer are not coextensive. That is, the self-exciting,self-sensing piezoelectric cantilever sensor is configured such that anentire surface of the non-piezoelectric layer is not coupled to anentire surface of the piezoelectric layer.

The sensitivity of the self-exciting, self-sensing piezoelectriccantilever sensor is due in part to utilizing the piezoelectric layer ofthe cantilever sensor for both actuation and sensing and theelectromechanical properties of the piezoelectric layer of theself-exciting, self-sensing piezoelectric cantilever sensor. Atresonance, the oscillating cantilever concentrates stress in thepiezoelectric layer toward a base portion of the self-exciting,self-sensing piezoelectric cantilever. This results in an amplifiedchange in the resistive component of the piezoelectric layer, and alarge shift in resonance frequency. Directing this stress to a portionof the piezoelectric layer having a low bending modulus (e.g., moreflexible) allows for exploitation of the associated shift in resonancefrequency to detect extremely small changes in mass of theself-exciting, self-sensing piezoelectric cantilever sensor. Forexample, if both the piezoelectric layer and the non-piezoelectric layerof a piezoelectric cantilever sensor are anchored at the same end (e.g.,potted in epoxy), the sensor is less sensitive to changes in massbecause the bending stress in the sensing piezoelectric layer proximalto the anchored end is lower compared to the case when only thepiezoelectric layer is anchored. This is because the bending modulus ofthe two combined layers is higher than the case of anchoring thepiezoelectric layer only. Bending modulus is the product of elasticmodulus and moment of inertia about the neutral axis. And, moment ofinertia is proportional to the cube power of thickness.

FIG. 1 is an illustration of a self-exciting, self-sensing piezoelectriccantilever sensor 12 comprising a piezoelectric portion 14 and anon-piezoelectric portion 16. Piezoelectric portions are labeled with anuppercase letter p (“P”), and non-piezoelectric portions are labeledwith the uppercase letters np (“NP”). The self-exciting, self-sensingpiezoelectric cantilever sensor 12 depicts an embodiment of anunanchored, overhang, self-exciting, self-sensing piezoelectriccantilever sensor. The self-exciting, self-sensing piezoelectriccantilever sensor 12 is termed “unanchored” because thenon-piezoelectric layer 16 is not attached to the base portion 20. Theself-exciting, self-sensing piezoelectric cantilever sensor 12 istermed, “overhang” because the non-piezoelectric layer 16 extends beyondthe distal tip 24 of the piezoelectric layer 14 to create an overhangingportion 22 of the non-piezoelectric layer 16. The piezoelectric portion14 is coupled to the non-piezoelectric portion 16 via adhesive portion18. The piezoelectric portion 14 and the non-piezoelectric portionoverlap at region 23. The adhesive portion 18 is positioned between theoverlapping portions of the piezoelectric portion 14 and thenon-piezoelectric portion 16. The piezoelectric portion 14 is coupled toa base portion 20.

The piezoelectric portion 14 can comprise any appropriate material suchas lead zirconate titanate, lead magnesium niobate-lead titanate solidsolutions, strontium lead titanate, quartz silica, piezoelectric ceramiclead zirconate and titanate (PZT), piezoceramic-polymer fibercomposites, or the like, for example. The non-piezoelectric portion 16can comprise any appropriate material such as glass, ceramics, metals,polymers and composites of one or more of ceramics, and polymers, suchas silicon dioxide, copper, stainless steel, titanium, or the like, forexample.

The self-exciting, self-sensing piezoelectric cantilever sensor cancomprise portions having any appropriate combination of dimensions.Further, physical dimensions can be non-uniform. Thus, the piezoelectriclayer and/or the non-piezoelectric layer can be tapered. For example,the length (e.g., L_(P) in FIG. 1) of the piezoelectric portion (e.g.,piezoelectric portion 14) can range from about 0.1 to about 10 mm. Thelength (e.g., L_(NP) in FIG. 1) of the non-piezoelectric portion (e.g.,non-piezoelectric portion 16) can range from about 0.1 to about 10 mm.The overlap region (e.g., overlap region 23) can range from about 0.1 toabout 10 mm in length. The width (e.g., W_(P) in FIG. 1) of thepiezoelectric portion (e.g., piezoelectric portion 14), and the width(e.g., W_(NP) in FIG. 1) of the non-piezoelectric portion (e.g.,non-piezoelectric portion 16), can range from about 0.1 mm to about 4.0mm. The width (e.g., W_(P) in FIG. 1) of the piezoelectric portion candiffer from the width (e.g., W_(NP) in FIG. 1) of the non-piezoelectricportion as well. The thickness of the (e.g., Tp in FIG. 1) of thepiezoelectric portion (e.g., piezoelectric portion 14), and thethickness (e.g., T_(NP) in FIG. 1) of the non-piezoelectric portion(e.g., non-piezoelectric portion 16), can range from about 0.1 mm toabout 4.0 mm. The thickness (e.g., T_(P) in FIG. 1) of the piezoelectricportion also can differ from the thickness (e.g., T_(NP) in FIG. 1) ofthe non-piezoelectric portion.

FIG. 2 is a cross-sectional view of the self-exciting, self-sensingpiezoelectric cantilever sensor 12 depicting electrode placement regions26 for electrodes operationally associated with the piezoelectricportion 14. Electrodes can be placed at any appropriate location on thepiezoelectric portion of the self-exciting, self-sensing piezoelectriccantilever sensor as indicated by brackets 26. For example, as shown inFIG. 3, electrodes 28 can be coupled to the piezoelectric portion 14within the base portion 20. Or, as depicted in FIG. 4, electrodes 32 canbe coupled to the piezoelectric portion 14 at any location not withinthe base portion 20 and not overlapped by the non-piezoelectric portion16. Electrodes need not be placed symmetrically about the piezoelectricportion 14. In an example embodiment, one electrode can be coupled tothe piezoelectric portion 14 within the base portion 20 and the otherelectrode can be coupled to the piezoelectric portion 14 not within thebase portion 20. Electrodes, or any appropriate means (e.g., inductivemeans, wireless means), can be utilized to provide an electrical signalto and receive an electrical signal from the piezoelectric portion 14.In an example embodiment, electrodes can be coupled to the piezoelectricportion 14 via a bonding pad or the like (depicted as elements 30 inFIG. 3 and elements 34 in FIG. 4). Example bonding pads can comprise anyappropriate material (e.g., gold, silicon oxide) capable ofimmobilization of a receptor material and/or an absorbent materialappropriate for use in chemical sensing or for bio-sensing.

Electrodes can be placed at any appropriate location. In an exampleembodiment, electrodes are operatively located near a location ofconcentrated stress in the piezoelectric layer 14. As described above,the sensitivity of the self-exciting, self-sensing piezoelectriccantilever sensor is due in part to advantageously directing(concentrating) the stress in the piezoelectric layer 14 and placingelectrodes proximate thereto. The configurations of the self-exciting,self-sensing piezoelectric cantilever sensor described herein (andvariants thereof) tend to concentrate oscillation associated stress inthe piezoelectric layer 14. At resonance, in some of the configurationsof the self-exciting, self-sensing piezoelectric cantilever sensor, theoscillating cantilever concentrates stress in the piezoelectric layer 14toward the base portion 20. This results in an amplified change in theresistive component of the piezoelectric layer 14, and a large shift inresonance frequency at the locations of high stress. Directing thisstress to a portion of the piezoelectric layer 14 having a low bendingmodulus (e.g., more flexible) allows for exploitation of the associatedshift in resonance frequency to detect extremely small changes in massof the self-exciting, self-sensing piezoelectric cantilever sensor.Thus, in example configurations of the self-exciting, self-sensingpiezoelectric cantilever sensor, the thickness of the piezoelectriclayer 14 located near the base portion 20 is thinner than portions ofthe piezoelectric layer 14 further away from the base portion 20. Thistends to concentrate stress toward the thinner portion of thepiezoelectric layer 14. In example configurations, electrodes arelocated at or near the locations of the oscillation associatedconcentrated stress near the base portion of the self-exciting,self-sensing piezoelectric cantilever sensor. In other exampleconfigurations of the self-exciting, self-sensing piezoelectriccantilever sensor electrodes are positioned proximate the location ofconcentrated stress in the piezoelectric layer regardless of theproximity of the concentrated stress to a base portion of theself-exciting, self-sensing piezoelectric cantilever sensor.

The self-exciting, self-sensing piezoelectric cantilever sensor can beconfigured in accordance with a plurality of configurations, some ofwhich are depicted in FIG. 5 through FIG. 16. It is to be understoodhowever, that the configurations depicted herein do not represent allpossible configurations, but rather a representative sample ofconfigurations of the self-exciting, self-sensing piezoelectriccantilever sensor. FIG. 5 is an illustration of an example configuration36 of an unanchored self-exciting, self-sensing piezoelectric cantileversensor wherein the distal end 40 of the piezoelectric portion 14 isflush with the distal end 38 of the non-piezoelectric portion 16. Theself-exciting, self-sensing piezoelectric cantilever sensor 36 is termed“unanchored” because the non-piezoelectric portion 16 is not attached tothe base portion 20. The piezoelectric portion 14 is coupled to thenon-piezoelectric portion 16 via adhesive portion 18. The adhesiveportion 18 is positioned between the overlapping portions of thepiezoelectric portion 14 and the non-piezoelectric portion 16. Thepiezoelectric portion 14 is coupled to a base portion 20.

FIG. 6 is an illustration of an example configuration 42 of anunanchored self-exciting, self-sensing piezoelectric cantilever sensorwherein the distal end 44 of the piezoelectric portion 14 extends beyondthe distal end 46 of the non-piezoelectric portion 16 and the proximateend 43 of the piezoelectric portion 14 extends beyond the proximate end45 of the non-piezoelectric portion 16. The piezoelectric portion 14 iscoupled to the non-piezoelectric portion 16 via adhesive portion 18. Theadhesive portion 18 is positioned between the overlapping portions ofthe piezoelectric portion 14 and the non-piezoelectric portion 16. Thepiezoelectric portion 14 is coupled to the base portion 20.

The self-exciting, self-sensing piezoelectric cantilever sensor also canbe configured to comprise multiple base portions. Example configurationsof the self-exciting, self-sensing piezoelectric cantilever sensorcomprising multiple base portions are depicted in FIG. 7 through FIG.14. Configuring the self-exciting, self-sensing piezoelectric cantileversensor to comprise multiple base portions is not intuitive because theexpectation of one skilled in the art would be that affixation of bothends of the self-exciting, self-sensing piezoelectric cantilever sensorwould provide a poor response as a result of the restrictions of thedisplacement of the self-exciting, self-sensing piezoelectric cantileversensor as a result of its affixation to the multiple base portions. Forconfigurations of the self-exciting, self-sensing piezoelectriccantilever sensor comprising two base portions, in an exampleembodiment, the stress of in the piezoelectric portion is measured,rather than the displacement of the piezoelectric portion. Configuringthe self-exciting, self-sensing piezoelectric cantilever sensor tocomprise two base portions provides a stable and robust sensor that canperform under relatively high media flow conditions and provideexcellent mass change sensitivity. Along with providing a mechanicallyrobust self-exciting, self-sensing piezoelectric cantilever sensor thatcan withstand a relatively wide range of media flow conditions withminimal determination in performance, configuring the self-exciting,self-sensing piezoelectric cantilever sensor to comprise two baseportions provides a fundamental frequency (e.g., greater than 100 kHz)that is three to four times higher than a cantilever sensor having asingle base portion and of similar dimensions.

FIG. 7 is an illustration of an example configuration 48 of an anchoredself-exciting, self-sensing piezoelectric cantilever sensor comprisingtwo base portions 20, 50. The self-exciting, self-sensing piezoelectriccantilever sensor 48 is termed “anchored” because the non-piezoelectricportion 16 is attached to the base portion 20. In the configurationdepicted in the self-exciting, self-sensing piezoelectric cantileversensor 48, both the proximate end 52 of the piezoelectric portion 14 andthe proximate end 54 of the non-piezoelectric portion 16 are attached tothe base portion 20. The piezoelectric portion and the non-piezoelectricportion can be attached to the base portion via any appropriate means.The distal end 58 of the non-piezoelectric portion 16 also is attachedto the base portion 50. The distal end 58 of the non-piezoelectricportion 16 extends beyond the distal portion 56 of the piezoelectricportion 14. The piezoelectric portion 14 is coupled to thenon-piezoelectric portion 16 via adhesive portion 18. The adhesiveportion 18 is positioned between the overlapping portions of thepiezoelectric portion 14 and the non-piezoelectric portion 16.

FIG. 8 is an illustration of an example configuration 60 of an anchoredself-exciting, self-sensing piezoelectric cantilever sensor comprisingtwo base portions 20, 50, wherein the piezoelectric portion 14 is notattached to either base portion 20 or base portion 50. In theconfiguration depicted in the self-exciting, self-sensing piezoelectriccantilever sensor 60, the proximate end 62 of the non-piezoelectricportion 16 is attached to the base portion 20 and the distal end 64 ofthe non-piezoelectric portion 16 is attached to the base portion 50. Theproximate end 62 of the non-piezoelectric portion 16 extends beyond theproximate end 66 of the piezoelectric portion 14 and the distal end 64of the non-piezoelectric portion 16 extends beyond the distal end 68 ofthe piezoelectric portion 14. The piezoelectric portion 14 is coupled tothe non-piezoelectric portion 16 via adhesive portion 18. The adhesiveportion 18 is positioned between the overlapping portions of thepiezoelectric portion 14 and the non-piezoelectric portion 16.

FIG. 9 is an illustration of an example configuration 70 of an anchoredself-exciting, self-sensing piezoelectric cantilever sensor comprisingtwo base portions 20, 50, comprising two piezoelectric portions 14, 72,and comprising two adhesive portions 18, 74. In the configurationdepicted in the self-exciting, self-sensing piezoelectric cantileversensor 70, the proximate end 76 of the piezoelectric portion 14 and theproximate end 78 of the non-piezoelectric portion 16 are attached to thebase portion 20. The distal end 80 of the piezoelectric portion 72 andthe distal end 82 of the non-piezoelectric portion 16 are attached tothe base portion 50. The proximate end 78 of the non-piezoelectricportion 16 extends beyond the proximate end 86 of the piezoelectricportion 72. The distal end 82 of the non-piezoelectric portion 16extends beyond the distal end 84 of the piezoelectric portion 14. Thedistal end 84 of the piezoelectric portion 14 and the proximate end 86of the piezoelectric portion 72 form a space 88 therebetween. Thepiezoelectric portion 14 is coupled to the non-piezoelectric portion 16via adhesive portion 18. The piezoelectric portion 72 is coupled to thenon-piezoelectric portion 16 via adhesive portion 74. The adhesiveportions 18 and 74 are positioned, respectively, between the overlappingportions of the piezoelectric portion 14 and the non-piezoelectricportion 16, and the piezoelectric portion 72 and the non-piezoelectricportion 16.

In various alternate example configurations of the configuration 70depicted in FIG. 9, only one of the piezoelectric portions 14, 72 isattached to a respective base portion 20, 50. For example, in oneexample configuration as depicted in FIG. 10, the piezoelectric portion14 is attached to the base portion 20 and the piezoelectric portion 72is not attached to the base portion 50. In another exampleconfiguration, as depicted in FIG. 11, the piezoelectric portion 72 isattached to the base portion 50 and the piezoelectric portion 14 is notattached to the base portion 20. In yet another example configuration,as depicted in FIG. 12, neither the piezoelectric portion 14 nor thepiezoelectric portion 72 is attached to a respective base portion 20,50. In the various example configurations in which a piezoelectric layercomprises multiple portions, electrodes can be attached to anyappropriate piezoelectric portion or portions. For example, in theexample configuration depicted in FIG. 9, FIG. 10, FIG. 11, and FIG. 12,electrodes can be attached to piezoelectric portion 14, piezoelectricportion 72, or a combination thereof.

FIG. 13 is an illustration of an example configuration 90 of an anchoredself-exciting, self-sensing piezoelectric cantilever sensor comprisingtwo base portions 20, 50, wherein the piezoelectric portion 14 isattached to the base portion 20 and the non-piezoelectric portion 16 isattached to the base portion 50. The piezoelectric portion 14 is coupledto the non-piezoelectric portion 16 via adhesive portion 18. Theadhesive portion 18 is positioned between the overlapping portions ofthe piezoelectric portion 14 and the non-piezoelectric portion 16. Thedistal end 98 of the non-piezoelectric portion 16 extends beyond thedistal end 96 of the piezoelectric portion 14. The proximate end 92 ofthe piezoelectric portion 14 extends beyond the proximate end 94 of thenon-piezoelectric portion 16.

FIG. 14 is an illustration of an example configuration 100 of ananchored self-exciting, self-sensing piezoelectric cantilever sensorcomprising two base portions 20, 50, wherein the non-piezoelectricportion 16 is not attached to either base portion 20 or base portion 50.In the configuration depicted in the self-exciting, self-sensingpiezoelectric cantilever sensor 100, the proximate end 102 of thepiezoelectric portion 14 is attached to the base portion 20 and thedistal end 104 of the piezoelectric portion 14 is attached to the baseportion 50. The proximate end 102 of the piezoelectric portion 14extends beyond the proximate end 106 of the non-piezoelectric portion 16and the distal end 104 of the piezoelectric portion 14 extends beyondthe distal end 108 of the non-piezoelectric portion 16. Thepiezoelectric portion 14 is coupled to the non-piezoelectric portion 16via adhesive portion 18. The adhesive portion 18 is positioned betweenthe overlapping portions of the piezoelectric portion 14 and thenon-piezoelectric portion 16.

FIG. 15 is an illustration of an example configuration 110 of anunanchored self-exciting, self-sensing piezoelectric cantilever sensorcomprising a piezoelectric portion 14 and a non-piezoelectric portion16, wherein the width, W_(P), of the piezoelectric portion is less thanthe width, W_(NP), of the non-piezoelectric portion 16. Theconfiguration 110 depicted in FIG. 15 is similar to the configuration 12depicted in FIG. 1, with the exception that W_(P) is less than W_(NP).According, the self-exciting, self-sensing piezoelectric cantileversensor 110 depicts an embodiment of an unanchored, overhang,self-exciting, self-sensing piezoelectric cantilever sensor. Thepiezoelectric portion 14 is coupled to the non-piezoelectric portion 16via adhesive portion (adhesive portion not shown in FIG. 15). Theadhesive portion is positioned between the overlapping portions of thepiezoelectric portion 14 and the non-piezoelectric portion 16. Thepiezoelectric portion 14 is coupled to a base portion 20.

FIG. 16 is an illustration of an example configuration 112 of anunanchored self-exciting, self-sensing piezoelectric cantilever sensorcomprising a piezoelectric portion 14 and a non-piezoelectric portion16, wherein the width, W_(P), of the piezoelectric portion is less thanthe width, W_(NP), of the non-piezoelectric portion 16, and wherein thedistal end 114 of the piezoelectric portion 14 extends beyond the distalend 116 of the non-piezoelectric portion 16 and the proximate end 118 ofthe piezoelectric portion 14 extends beyond the proximate end 120 of thenon-piezoelectric portion 16. The configuration 112 depicted in FIG. 16is similar to the configuration 42 depicted in FIG. 6, with theexception that W_(P) is less than W_(NP). The piezoelectric portion 14is coupled to the non-piezoelectric portion 16 via adhesive portion(adhesive portion not shown in FIG. 16). The adhesive portion ispositioned between the overlapping portions of the piezoelectric portion14 and the non-piezoelectric portion 16. The piezoelectric portion 14 iscoupled to the base portion 20.

FIG. 17 is a flow diagram of an example process for detecting an analyteutilizing the self-exciting, self-sensing piezoelectric cantileversensor. The self-exciting, self-sensing piezoelectric cantilever sensoris provided at step 120. The self-exciting, self-sensing piezoelectriccantilever sensor can be configured in accordance with the descriptionsprovided above, or configured in accordance with any appropriate variantis thereof. The self-exciting, self-sensing piezoelectric cantileversensor is prepared to receive an analyte at step 122. In an exampleembodiment, an analyte attractor is applied to the non-piezoelectricportion of the self-exciting, self-sensing piezoelectric cantileversensor. The attractor is specific to an analyte. Thus the attractor willattract a target analyte and not attract other substances. For example,the non-piezoelectric portion of the self-exciting, self-sensingpiezoelectric cantilever sensor can comprise an attractor for attractingbioterrorism agents, such as Bacillus anthracis, food-borne pathogens,such as E. coli, pathogens in food and water, cell types in body fluids(e.g., circulating tumor cells), biomarkers in body fluids (e.g.,proteins that mark specific pathophysiology-alpha-fetoprotein,beta-2-microglobulin, bladder tumor antigen, breast cancer markerCA-15-3, and other CAs (cancer antigens), calcitonin, carcinoembryonicantigen, and others), markers of explosives such as trinitrotoluene,dinitrotoluene, airborne and waterborne toxins, biological entities,such as a protein, or a combination thereof, for example.

The self-exciting, self-sensing piezoelectric cantilever sensor isexposed to a medium at step 124. The medium can comprise any appropriatemedium, such as a liquid, a gas, a combination of a liquid and a gas, ora vacuum, for example. The medium can exhibit a wide variety of flowconditions. If a target analyte is present in the medium, the targetanalyte will accumulate on the non-piezoelectric portion of theself-exciting, self-sensing piezoelectric cantilever sensor that hasbeen treated with the attractor. As described above, accumulation (e.g.,binding) of the target analyte on the non-piezoelectric portion of theself-exciting, self-sensing piezoelectric cantilever sensor will resultin a change in stiffness of the self-exciting, self-sensingpiezoelectric cantilever sensor and/or an increase the mass of theself-exciting, self-sensing piezoelectric cantilever sensor, which willdecrease the resonance frequency of the self-exciting, self-sensingpiezoelectric cantilever sensor.

The resonance frequency of the self-exciting, self-sensing piezoelectriccantilever sensor is measure at step 126. The resonance frequency can bemeasured by any appropriate means, such as an operational amplifier, animpedance analyzer, a network analyzer, an oscillator circuit, or thelike, for example. When the piezoelectric material of the piezoelectricportion of the self-exciting, self-sensing piezoelectric cantileversensor is excited, the non-piezoelectric portion of the self-exciting,self-sensing piezoelectric cantilever sensor flexes to accommodate thestrain caused in the piezoelectric material. When the frequency ofexcitation is the same as the natural frequency of the underlyingmechanical structure, resonance occurs.

The measured resonance frequency is compared to a baseline resonancefrequency at step 128. The baseline resonance frequency is the resonancefrequency of the self-exciting, self-sensing piezoelectric cantileversensor having no analyte accumulated thereon. If a difference infrequency (frequency shift) between the measured resonance frequency andthe baseline resonance frequency is not measured (at step 130), it isdetermined, at step 132, that no analyte is detected. If a difference infrequency between the measured resonance frequency and the baselineresonance frequency is measured (at step 130), it is determined, at step134, that an analyte is detected, i.e., an analyte is present in themedium. At step 136, the amount of mass of the analyte that hasaccumulated on the non-piezoelectric portion of the self-exciting,self-sensing piezoelectric cantilever sensor is determined in accordancewith the frequency shift measured at step 130.

Various experiments have been conducted utilizing various configurationsof the self-exciting, self-sensing piezoelectric cantilever sensor. FIG.18 is a plot 137 of an example resonance spectrum of the configuration12 of the self-exciting, self-sensing piezoelectric cantilever sensor,depicted in FIG. 1, operated in air. The width, W_(P), and the width,W_(NP), were each approximately 2 mm. The plot 137 shows the phase angle(between the excitation voltage and the excitation current) versusexcitation frequency, at an excitation voltage of 100 mV. The firstresonance frequency mode 140 occurred approximately between 150 and 200kHz and the second resonance frequency mode 142 occurred between 250 and300 kHz. The resonance spectrum shows higher order characteristic peaksat approximately 980 kHz, 2.90 MHz and 4.60 MHz.

Quality factors were determined as a ratio of the resonant frequency tothe peak width at half the peak height. As a result, the quality factoris a measure of the sharpness of the resonant peaks. Experimentation hasshown that the quality factor of the self-exciting, self-sensingpiezoelectric cantilever sensor does not decrease significantly when thesensor is placed in different environments ranging from vacuum to liquidflow environments. Also, experimentation has shown that the Q values forthe various configurations of the self-exciting, self-sensingpiezoelectric cantilever sensor typically range between 10 and 70,depending upon the respective frequency mode where the peak is detected.The various configurations of the self-exciting, self-sensingpiezoelectric cantilever sensor, when used in vacuum, air, and viscousenvironments, including flows, typically did not have more than a20%-35% decrease in Q value. This relatively small loss in the overallvalue of the quality factor reflects the ability of the self-exciting,self-sensing piezoelectric cantilever sensor to accurately detectchemicals and various biological items in viscous environments,including water and bloodstreams.

Experimentation has shown that the sensitivity of the self-exciting,self-sensing piezoelectric cantilever sensor is a function of thedimensions thereof. Specific changes in the geometry of theself-exciting, self-sensing piezoelectric cantilever sensor enhanced thesensor's mass change sensitivity, and thus, the sensor's response to thedetection of low concentration of analyte. The resonance spectrum, aplot of phase angle versus excitation frequency, in air, showed dominantbending mode resonant peaks at 102±0.05, 970±0.05, and 1810±0.05 kHz,respectively. By changing the geometry of the of the self-exciting,self-sensing piezoelectric cantilever sensor, the sensor's resonancecharacteristics were enhanced. The corresponding bending resonant modesoccurred at higher frequencies and had larger phase angles, suggestingthat resonant peaks of the self-exciting, self-sensing piezoelectriccantilever sensor are more sensitive and are less dampened.

In an example experiment, the mass change sensitivity of theself-exciting, self-sensing piezoelectric cantilever sensor wasmeasured. A known mass of paraffin wax was added to a glass surface ofthe self-exciting, self-sensing piezoelectric cantilever sensor and thechange in resonant frequency was used to compute the mass sensitivity,expressed in g/Hz. Direct measurement was made of the mass changesensitivity in liquid; as well as the ratio of known mass to the changein resonant frequency in liquid before and after mass was added. Themass sensitivity of the resonant mode investigated under liquid wasdetermined to be 1.5×10⁻¹⁵ g/Hz.

1. A method for detecting an analyte, the method comprising: exposing atleast a portion of a non-piezoelectric layer of a cantilever sensor to amedium, the cantilever sensor comprising: a piezoelectric layercomprising a proximate end and a distal end; a base portion coupled tothe proximate end of the piezoelectric layer; the non-piezoelectriclayer, comprising a proximate end and a distal end, wherein: at least aportion of the piezoelectric layer is coupled to at least a portion ofthe non-piezoelectric layer such that the piezoelectric layer and thenon-piezoelectric layer are not coextensive; and the base portion is notattached to the non-piezoelectric layer; electrodes operativelyassociated with the piezoelectric layer. measuring, via the electrodes,a resonance frequency of the sensor; comparing the measured resonancefrequency with a baseline resonance frequency; when the measuredresonance frequency differs from the baseline resonance frequency,determining that an analyte is present in the medium.
 2. A method inaccordance with claim 1, wherein the baseline resonance frequency is aresonance frequency of the sensor having no analyte accumulated thereon.3. A method in accordance with claim 1, wherein medium comprises one ofa liquid, a gas, and a vacuum.
 4. A method in accordance with claim 1,wherein the analyte comprises at least one of a bioterrorism agent, afood-borne pathogen, a water pathogen, a cell type in a body fluids, abiomarker in a body fluid, an indication of an explosive material, anairborne toxin, a waterborne toxin, and a biological entity.
 5. A methodin accordance with claim 35, further comprising determining an amount ofanalyte accumulated on the sensor in accordance with the differencebetween the measured resonance frequency and the baseline resonancefrequency.
 6. A method in accordance with claim 1, further comprisingdetermining a change in an amount of mass of an analyte accumulated onthe sensor in accordance with the difference between the measuredresonance frequency and the baseline resonance frequency, wherein a 1Hertz difference between the measured resonance frequency and thebaseline resonance frequency is indicative of a change is mass of about100 attograms.
 7. A method in accordance with claim 1, furthercomprising detecting a presence of an analyte in the medium, wherein theanalyte comprises at least one of a protein, a lipoprotein, DNA, and RNAin the medium at a concentration of 1 femtograms per mL.
 8. A method inaccordance with claim 1, further comprising detecting a presence of ananalyte in the medium, wherein the analyte comprises a pathogen in themedium at a concentration of 1 pathogen per mL.
 9. A method inaccordance with claim 1, wherein a difference in the measured resonancefrequency and the baseline resonance frequency is indicative of a stressin the piezoelectric layer.
 10. A method in accordance with claim 1,wherein oscillation associated stress is concentrated at a location inthe piezoelectric layer, the method further comprising positioning theelectrodes proximate to the location of the concentrated stress.